There are some NEC and Mitsubishi 3.5" drives that have a fair number
of additional signals on the odd-numbered connector pin side--usually
used on CNC and some lab equipment on 9801-series architectures.
When replacing these, we've found that any old 3.5" drive won't do--
an exact replacement is necessary.
I'm surprised no one mentioned 3.5" DSED testing.
Cheers,
Chuck